docs: def get_all_formatters():
      """Return a generator for all formatter classes."""
      # NB: this returns formatter classes, not info like get_all_lexers().
      for info in itervalues(FORMATTERS):
          if info[1] not in _formatter_cache:
              _load_formatters(info[0])
          yield _formatter_cache[info[1]]
      for _, formatter in find_plugin_formatters():
          yield formatter